Coral Reefs
Coral reefs are underwater structures built by colonies of tiny marine animals called coral polyps. Each polyp secretes a hard skeleton of calcium carbonate, and over many generations these skeletons accumulate into the massive limestone formations that make up a reef. Reefs grow slowly, often over thousands of years, in warm, clear, shallow tropical and subtropical seas where sunlight can reach the corals.
Many reef-building corals owe their success to a partnership with microscopic algae called zooxanthellae that live within their tissues. The algae photosynthesise and supply much of the coral's food, while the coral provides shelter and nutrients. This relationship depends on stable, warm but not overheated water; when corals are stressed, they may expel the algae and turn white, a process known as coral bleaching. Reefs are among the most biologically diverse environments in the ocean, providing food and habitat for fish, molluscs, crustaceans and countless other organisms, which is why they are often compared to rainforests in their richness.
Beyond their ecological value, coral reefs protect coastlines from wave erosion, support fisheries and draw the interest of physical oceanography and marine ecology, fields that also study features such as deep-sea hydrothermal vents. Reefs are sensitive to changes in water temperature, acidity and pollution, and many are protected within marine sanctuaries to help conserve their ecosystems.
Frequently asked questions
- Are corals plants or animals?
- Corals are animals. Each coral is a colony of small polyps related to sea anemones and jellyfish, though many host photosynthetic algae inside their tissues.
- What causes coral bleaching?
- Bleaching occurs when stressed corals expel the symbiotic algae that give them colour and much of their food, often because water has become too warm. Prolonged bleaching can kill the coral.
- Why are coral reefs important?
- They support enormous marine biodiversity, sustain fisheries, protect shorelines from waves and erosion, and are valued for tourism and scientific study.
